Field of the Invention
The present invention relates generally to burnable absorber rod manufacture and, more particularly, is concerned with an apparatus and method for welding an end plug on an inner tube of a burnable absorber rod.
Description of the Prior Art
The incorporation of burnable absorbers, also referred to as burnable poisons, in nuclear fuel assemblies has been recognized in the nuclear power generating field as an effective means of increasing fissionable material capacity and thereby extending reactor core life. Burnable absorbers, being parasitic neutron-capturing elements, are used either uniformly mixed with the fissionable material, deposited as a coating on the exterior of nuclear fuel pellets containing fissionable material, or placed as separate elements in the reactor core.
U.S. Pat. No. 3,510,398 to Wood, assigned to the assignee of the present invention, discloses an example of a separate burnable absorber rod. In the rod of the Wood patent, the burnable poison, boron, is found in a hollow stainless steel clad borosilicate glass tube contained within an outer cladding tube.
U.S. Pat. No. 4,460,540 to Funk et al, also assigned to the assignee of the present invention, discloses another example of a separate burnable absorber rod. In the rod of the Funk et al patent, hollow inner and outer tubes in a concentric arrangement provide an annular space therebetween which contains annular burnable poison pellets therein. The annular space is sealed at the opposite ends of the concentric hollow inner and outer tubes by end plugs. Reactor core coolant water is allowed to pass through an aperture in the bottom end plug upwardly through the inner tube and out of an aperture in the top end plug.
U.S. Pat. No. 4,626,404 to Chubb, also assigned to the assignee of the present invention, discloses still another example of a separate burnable absorber rod generally similar to the rod of the Funk et al patent. However, in the rod of the Chubb patent, the annular space between the concentric inner and outer tubes contains a metal cylindrical tube having burnable absorber material deposited over its outer surface.
Another known absorber rod generally similar to the rods of the Funk et al and Chubb patents employs a bottom end plug having an outer generally frusto-conical head portion and three generally cylindrical neck portions. The end plug also has a central passage for allowing flow of reactor coolant therethrough. The neck portions are tandemly arranged and of increasingly smaller diameter than the maximum diameter of the head portion proceeding away from the head portion from the outer one to the inner one of the neck portions. As a result of the diameter differences between the three neck portions themselves and also with respect to the outer head portion, an abutment or shoulder is formed at each transition from one neck portion to the next and to the outer head portion.
The outside diameter of the inner neck portion is slightly less than the inside diameter of the inner tube of the rod so that the end portion of the tube will slip over the inner neck portion and abut against the inner one of the shoulders. The same relationships hold with respect to the outside diameters of the middle and outer ones of the neck portions and the inside diameters of the middle and outer tubes of the rd so that the end portions of the middle and outer tubes will slip over the corresponding middle and outer neck portions and abut against the middle and outer ones of the shoulders. Typically, the end portions of the inner and outer tubes are welded to the end plug at the inner and outer end plug shoulders.
The first weld formed is the one found between the end portion of the inner tube of the rod and the inner shoulder between the inner and middle neck portions of the end plug. In the past, this weld has been formed by holding the rod stationary and orbiting or revolving a welding apparatus about the end plug. The welding apparatus employed a clam shell-type of housing or weld chamber for holding the tube with the end plug thereon. The weld chamber opened to allow placement of the larger diameter head portion of the end plug therein past a welding electrode and then closed to begin the welding operation.
Even though care was ordinarily taken to place the end plug in the housing without touching the electrode, bumping of the electrode would inadvertently occur from time to time. Such slight contact with the electrode frequently resulted in sufficient misalignment between it and the end plug that a girth weld could not be accurately formed at the interface of the end of the tube and the inner shoulder of the end plug.
One alternative approach to welding end plugs to tubes is disclosed in U.S. Patents to Fink et al (3,725,635), Duncan et al (4,075,454) and Yeo (4,188,521), which are all assigned to the same assignee as the present invention. The approach of these patents is to insert the tube with the end plug thereon into a welding chamber in alignment with a stationary welding electrode and then rotate the tube and plug as a girth weld is formed at the interface between the end plug and tube end. However, this approach does not appear to provide a solution to the problem of how to reliably insert a larger diameter head portion of an end plug past a welding electrode without bumping it in view that the electrode needs to be positioned close enough to the smaller diameter tube end to complete a girth weld between it and the end plug.
Consequently, a need still exists for a different approach to the end plug welding operation which will allow the larger diameter head portion of an end plug fitted into a tube to reliably clear the welding electrode as the end plug is being inserted axially into the weld chamber.